In oil and gas exploration it is desirable to understand the structure and properties of the geological formation surrounding a borehole, in order to determine if the formation contains hydrocarbon resources (oil and/or gas), to estimate the amount and producibility of hydrocarbon contained in the formation, and to evaluate the best options for completing the well in production. A significant aid in this evaluation is the use of wireline logging and/or logging-while-drilling (LWD) measurements of the formation surrounding the borehole (referred to collectively as “logs” or “log measurements”). Typically, one or more logging tools are lowered into the borehole and the tool readings or measurement logs are recorded as the tools traverse the borehole. These measurement logs are used to infer the desired formation properties.
In recent years nuclear magnetic resonance (NMR) logging has become very important for purposes of formation evaluation and is one of the preferred methods for determining formation parameters. Improvements in the NMR logging tools, as well as advances in data analysis and interpretation allow log analysts to generate detailed reservoir description reports, including clay-bound and capillary-bound related porosity, estimates of the amounts of bound and free fluids, fluid types (i.e., oil, gas and water), permeability and other properties of interest.
NMR tools used in practical applications include, for example, the centralized MRIL® tool made by NUMAR Corporation, a Halliburton company, and the sidewall CMR tool made by Schlumberger. The MRIL® tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications including: “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,” by Miller, Paltiel, Gillen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sep. 23–26, 1990; “Improved Log Quality With a Dual-Frequency Pulsed NMR Tool,” by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sep. 25–28, 1994. Certain details of the structure and the use of the MRIL® tool, as well as the interpretation of various measurement parameters are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164; 6,051,973; 6,107,796 and 6,111,408. The structure and operation of the Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 4,939,648; 5,055,787 and 5,055,788 and further in “Novel NMR Apparatus for Investigating an External Sample,” by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466–485, 1992; and “An Improved NMR Tool Design for Faster Logging,” D. McKeon et al., SPWLA 40th Annual Logging Symposium, May–June 1999. The content of the above patents is hereby expressly incorporated by reference for all purposes, and all non-patent references are incorporated by reference for background.
NMR tools of the type discussed above generally measure the time for hydrogen nuclei present in the earth formation to realign their spin axes, and consequently their bulk magnetization, either with an externally applied magnetic field, or perpendicularly to the magnetic field, after momentary reorientation due to the application of specific radio frequency (RF) pulses. The externally applied magnetic field is typically provided by a magnet disposed in the tool. The spin axes of the hydrogen nuclei in the earth formation are, in the aggregate, caused to be aligned with the magnetic field induced in the earth formation by the magnet. The NMR tool includes an antenna positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces a magnetic field in the earth formation orthogonal to the field induced by the magnet. The RF pulse has a duration predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicular both to the orthogonal magnetic field induced by the RF pulse and to the externally applied magnetic field. After the pulse ends, the nuclear magnetic moment of the hydrogen nuclei gradually relax, i.e., return to their alignment with the externally applied magnetic field; at the same time an antenna, which is typically the same as the one used by the initial pulse, is electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei.
An actual NMR measurement involves a plurality of pulses grouped into pulse sequences, most frequently of the type known in the art as Carr-Purcell-Meiboom-Gill (CMPG) pulsed spin echo sequences. As known in the art, each CPMG sequence consists of a 90-degree (i.e., π/2) pulse followed by a large number of 180-degree (i.e., π) pulses. The 90-degree pulse rotates the proton spins into the transverse plane and the 180-degree pulses generate a sequence of spin echoes by refocusing the transverse magnetization after each spin echo. A wait time interval (TW) of approximately 0.5–10 sec is used first to allow for polarization of the formation by the tool's static magnetic field. Then, a CPMG pulse-echo train is executed, consisting of an excitation pulse and an alternating sequence of refocusing pulses. Following each pair of excitation pulse and a refocusing pulse, an acquisition window is applied next. Complex data points from such a pair of echo trains are co-added on an echo-by-echo basis to remove certain artifacts and to enhance the NMR signal, as known in the art. More pairs may be added to enhance the signal-to-noise ratio. The echo train, consisting of a superposition of exponentially decaying signals is then submitted to a processor, which calculates the underlying decay modes of the received NMR echo signal.
One of the key information sought from the NMR logging data is the T2 decay time distribution, which is related to the pore size distribution of the rock formation. From the T2 decay time distribution one can estimate, for example, the capillary bound water volume. The T2 distribution is obtained by inversion of the echo train data, using well-known methods as described, for example, in U.S. Pat. No. 5,517,115 to the assignee of the present application.
It is also well-known in the art that inversion of NMR echo trains is greatly affected by noise. FIG. 1 illustrates a single-event echo train. In order to reduce the effect of noise and increase the signal-to-noise ratio (SNR), stacking of multiple echo train data is commonly used, by assuming that noise is of random nature. Thus, for example, the average echo train from 10 events is affected with much less noise than the echo train from a single event. (In general, if N is the number of stacked signals, the SNR of the averaged signal improves as approximately sqrt(N)). Various methods exist to perform stacking such as, for example, using two pulse sequences of opposite phase that cancel electronic offsets and 180-degree ringing. Such a pair of pulse sequences is called a phase-alternated pair (PAP).
In actual logging, the tool is moving at a certain speed, and echo train from each event is not collected at the same logging location or depth. When the logging speed is slow, the tool moves only a short distance. Hence one may assume that the earth formation is of nearly constant property over the short distance, and stacking of multiple events from different depths may be acceptable. However, when the logging speed is relatively fast, the tool moves over longer distances and the formation properties may change significantly over the distance. As a result, stacking of echo train data from multiple events, as generally used in the prior art, reduces apparent spatial resolution of measuremenets beyond the intrinsic resolution of the tool.
In particular, in high-signal formations (e.g., high porosity, oil-or water-filled rocks), where the logging speeds can be comparatively fast, the effective vertical resolution of the NMR log becomes dominated by the tool movement during a single PAP. For example, with a recovery time of 10 seconds between echo-trains in a PAP, with PAPs acquired at all possible frequencies, the elapsed time between the first echo-train in the first-frequency PAP and the second echo-train for the last-frequency PAP, is close to 20 seconds. At a logging speed of 900 ft/hr (15 ft/min), the MRIL tool will move approximately 5 feet during this measurement: when combined with the inherent vertical resolution of the antenna (which is approximately 2 feet), the effective vertical resolution becomes roughly 7 feet.
Enhancing the resolution of the logs is a significant problem, because subsurface formations are generally heterogeneous, so that porosity, saturation and lithology vary with position. A common example of heterogeneity is the presence in the formation of geological layers, or beds. Because logging tools have a nonzero volume of investigation, more than one layer may lie within the volume of investigation of a tool. In such cases, the petrophysical evaluation of one layer may be distorted by the presence of another layer falling within the larger volume of investigation of the tool. The above phenomenon leads to a specific problem in the analysis of subsurface formations that include one or more underground layers, especially when the layers are thin compared with the vertical resolution of the measuring tool. Such layers have become subject to significant commercial interest because of their production potential. Any knowledge about the composition and properties of such layered formations that helps better estimate their production potential has thus become increasingly valuable. It is clear therefore that any mechanism that for a given SNR supported by the formation can increase the vertical resolution of the tool without decreasing the logging speed is highly desirable.